Heretofore, it has been known to use, as a co-catalyst of a three-way catalyst for treatment of exhaust gas discharged from automobiles, oxides based on CeO2 (hereinafter, also referred to as “ceria”) excellent in performance on discharge and storage of oxygen. Here, the three-way catalyst is a catalyst having a function to remove carbon monoxide, hydrocarbons and nitrogen oxide from the exhaust gas generated with combustion of an internal combustion engine, by oxidizing or reducing them to carbon dioxide, water and nitrogen, respectively. Further, it has also been known to use a ceria catalyst as a catalyst for removing particulate matter (particulates) in the exhaust gas by oxidizing them to carbon dioxide and water.
If catalysts having such properties are developed, it is possible to reduce the amount of rare metals (such as Pt, Rh and Pd) as a rare resource which have been used as a catalyst material, whereby resource saving can be expected.
A solid solution (see e.g. Patent Document 1) having a rare-earth element such as lanthanum or zirconium solid-solved in ceria, and a solid solution (see e.g. Non-Patent Document 1) having cerium solid-solved in ZrO2 (hereinafter, also referred to as zirconia), are recently proposed as one of the ceria catalysts showing such properties.
As a process for producing such a solid solution, usually, a solution method such as an impregnation method or a coprecipitation method is employed. With reference to preparation of a solid solution having zirconium solid-solved in ceria as an example, the impregnation method is carried out in such a manner that a ceria powder is immersed in a solution containing zirconium, and the resulting immersed powder is subjected to heat treatment to obtain an oxide (see e.g. Patent Document 1). On the other hand, the coprecipitation method is carried out in such a manner that a solution containing cerium and zirconium is prepared, precipitates (coprecipitates) containing cerium and zirconium is obtained e.g. by controlling the pH of the solution, and then the coprecipitates are subjected to heat treatment to obtain an oxide (see e.g. Patent Document 2).
In the impregnation method, the particle size of a solid solution to be produced is likely to be controlled by the particle size of a raw material, and is likely to be affected particularly by the particle size of ceria particles as a raw material. Accordingly, in a case where the particle size of the raw material is large or non-uniform, it is impossible to obtain fine particulate particles having uniform particle size and chemical composition. On the other hand, in the coprecipitation method, it is difficult to obtain coprecipitates containing cerium and zirconium in a desired composition because constituent elements of the raw materials precipitate at respective different pHs. Therefore, for sufficient progress of solid solution formation, complicated operations are essential in selection of additives, control of reaction conditions, and so on.
Further, in either method, a heat treatment is essential and a resulting product will undergo grain growth by heat, and therefore it is not easy to obtain a ceria-zirconia solid solution in the form of fine particulates (e.g., fine particles with an average primary particle size of at most 200 nm). In addition, the crystallinity of fine particles to be obtained is also low.
In order to solve the problems, Patent Document 3 proposes a process for producing a ceria-zirconia solid solution by pulverizing a ceria powder and a zirconium compound in the presence of pulverization media, under conditions of friction of pulverization media with one another and/or pulverization media and a member of a pulverization apparatus. It is possible to obtain a fine particulate ceria-zirconia solid solution by such a method, but since an operation to let zirconia contained in the pulverization media and/or the member of the pulverization apparatus be solid-solved in ceria is essential, reaction tends to take time, such being problematic.
Patent Document 1: JP-A-4-55315 (Claims)
Patent Document 2: Japanese Patent No. 3341973 (Claims)
Patent Document 3: JP-A-8-333116 (Claims)
Non-Patent Document 1: Journal of the Society of Powder Technology (Vol. 41, No. 3, 218-223, 2004)